Hydrochemistry of rivers in an acid sulphate soil hotspot area in western Finland

Abstract

During heavy rains and snow melting, acid sulphate (AS) soils on the coastal plains of Finland are flushed resulting in discharge of acidic and metal-rich waters that strongly affect small streams. In this study, the impact of AS soils occurrence and hydrological changes on water quality were determined for 21 rivers (catchment sizes between 96-4122 km 2 ) running through an AS soil hotspot area in western central Finland. Water samples, collected at the outlet, during eight selected events, were analysed for pH, dissolved organic carbon, electrical conductivity (EC) and 32 chemical elements. Based on the correlation with percentage arable land in the catchments (a rough estimate of AS soil occurrences, as up to 50% of the arable land is underlain with these soils), it was possible to categorize variables into those that are enriched in runoff from such land, depleted in runoff from such land (only one element), and not affected by land-use type in the catchments. Of the variables enriched in runoff from arable land, some were leached from AS soils during high-water flows, in particular (aluminium, boron, beryllium, cadmium, cobalt, copper, lithium, manganese, nickel, sulphur, silicon, thorium, thallium, uranium, and zinc) and others occurred in highest concentrations during lower flows (calcium, EC, potassium, magnesium, sodium, rubidium and strontium). Molybdenum and phosphorus were not leached from AS soils in larger amounts than from other soils and thus related to other factors connected to the arable land. Based on the concentrations of potentially toxic metals derived from AS soils, the 21 rivers were ranked from the least (Lestijoki River, Lapväärtinjoki River and Perhonjoki River) to the most (Sulvanjoki River, Vöyrinjoki River and Maalahdenjoki River) heavily AS soil impacted. It has been decided that Vöyrinjoki is to be dredged along a ca. 20 km distance. This is quite alarming considering the high metal concentrations in the river.

Impact of isostatic land uplift and artificial drainage on oxidation of brackish-water sediments rich in metastable iron sulfide

“…In the drain water, the concentrations of Co, Ni and Zn were high, but those of As and Mo low, consistent with the geochemical features of the AS soil (site D) (Figs. 5 and 7) and in line with the patterns in other AS soils (Å strö m and Bjö rklund, 1997;Å strö m, 1998;Sohlenius and Ö born, 2004;Nordmyr et al, 2006) and waters affected by these soils (Å strö m and Spiro, 2000;Roos and Å strö m, 2005). Eventually this load reaches the estuary, where part of the trace metals are likely to precipitate with pyrite as shown at sites A and B, and possibly also with metastable iron sulfide probably forming further out in the estuary.…”

“…5) is most likely the estuarine water (cf. Macdonald et al, 2004;Nordmyr et al, 2007) contaminated by the metal load from Kyrö njoki River (Roos and Å strö m, 2005) that drains AS soils further inland. At site C, the obvious source is the ripening and acidic thin soil layer above.…”

“…It is also possible that small amounts of SO 2 may form at pyrite oxidation, which would result in sulfur losses to the atmosphere (Brunner et al, 2008). The overall result of the oxidation reactions is extensive development of environmentally unfriendly acidic (pH <4) soils, referred to as acid sulfate (AS) soils, which produce acidic and metal-rich runoff (Å strö m and Spiro, 2000;Joukainen and Yli-Halla, 2003;Roos and Å strö m, 2005;Nordmyr et al, 2006) often leading to fish kills (e.g. Hudd, 2000;Urho, 2002).…”

Characterization of acid sulfate soils and assessing their impact on a humic boreal lake

“…5) are clear indicators of a.s. soils in mid-western Finland (Österholm and Åström, 2004;Roos and Åström, 2005) showing that there is an obvious significant influence of a.s. soils on the near-field streams. Titanium, Fe, Cr and V are known to be leached from a.s. soils in a lesser extent (Österholm and Åström, 2002;Roos and Åström, 2005). As expected, these elements are also less enriched in the study area, and show a weak correlation to sulfate (e.g.…”

“…Zink, Ni, Mn, Co, Al, and EC in rivers in Larsmo and Öja area are clearly higher than the median concentrations in Lappfjärd River, one of the few rivers in mid-western Finland that is relatively unaffected by runoff from a.s. soils (Roos and Åström, 2005) (Table 4). Concentrations of Cd, Cu and Fe are, however, higher only in the most acidic rivers in the study area.…”